The present invention relates generally to a knocking control method for suppressing a knocking phenomenon in an internal combustion engine (hereinafter also referred to as an engine for short) such as a gasoline engine for a motor vehicle as well as an apparatus for carrying out the same. More particularly, the invention is concerned with an improved knocking control method and apparatus for an engine which allow failure or fault of a knocking sensor to be detected with high reliability, to thereby ensure a fail-safe operation of the engine knocking control system.
In general, the internal combustion engine such as a gasoline engine for a motor vehicle includes a plurality of cylinders in each of which a fuel gas mixture is compressed and undergoes combustion at an optimal timing. In this conjunction, there has already been proposed and used widely in practical applications a microcomputer-based engine control unit (also known as ECU in abbreviation) for the purpose of controlling optimally the ignition timings as well as the sequence of fuel injections in association with the individual engine cylinders.
In connection with such engine operation control, it is known that when the ignition timing (usually given in terms of angular crank position or crank angle) is controlled to advance excessively, abnormal fuel combustion may take place, resulting in generation of vibrations or shock referred to as knocking of such magnitude which may eventually damage or injure the engine cylinders. In order to avoid such unwanted event, it is necessary to perform ignition timing control in such a manner that upon detection of abnormal vibrations or knocking, the ignition timing is shifted in a direction to afford an appropriate retard to the time point or timing at which fuel combustion takes place within the knocking cylinder.
For a better understanding of the background of the present invention, an engine knocking control system known heretofore will be described in some detail by reference to FIG. 5 which is a block diagram showing the general arrangement of a known knocking control system.
In FIG. 5, a reference numeral 1 denotes a knocking sensor installed in association with each or a set of the cylinders of an internal combustion engine. The knocking sensor 1 may be composed of a piezoelectric element or the like which is capable of detecting the vibrations or knocking of the associated cylinder in the form of an electric signal.
An output signal A of the knocking sensor 1 is supplied to a knocking detection circuit denoted generally by a reference numeral 2. The knocking detection circuit 2 comprises a filter 21 having such a filtering characteristic as to pass therethrough only the frequency components which are peculiar to the knocking phenomenon (e.g., 7 kHz), a gate 22 for allowing the output signal of the filter 21 to pass therethrough periodically at a predetermined timing, a background level (BGL) generator 23 for generating a background level signal BGL on the basis of a signal derived by averaging an output signal A' of the gate 22, a comparator 24 for comparing the output signal A' of the gate 22 with the background level signal BGL for thereby producing an output signal of "ON" level when the gate output level A' exceeds the background level BGL, and an integrator 25 for integrating the output signal of the comparator 24. The output signal of the integrator 25 is then supplied to an analogue to digital (A/D) converter 3 to be converted to a digital signal V.sub.R.
The digital signal V.sub.R is supplied to an engine control unit (ECU in abbreviation) 4 which may be constituted by a microcomputer which is programmed to perform ignition timing control for each of the engine cylinders on the basis of the output signal V.sub.R of the A/D converter 3 while supplying a masking pulse signal M to the gate 22 and a reset signal R to the integrator 25, respectively, for the purposes which will be described hereinafter. Further, the engine control unit or controller 4 includes an ignition retard control processor 45 for arithmetically determining an angle of retard for which the ignition timing is to be delayed for suppressing the knocking, thereby producing a retard control angle signal .theta..sub.R for controlling the amount of retard to be applied to the ignition timing on the basis of the digital signal V.sub.R outputted from the A/D converter 3.
Next, referring to a waveform diagram shown in FIG. 6, description will be made of operations performed by the known engine knocking control system shown in FIG. 5.
Normally, in each of the cylinders of the internal combustion engine, ignition takes place at a timing corresponding to a crank angle or position which advances approximately by 5.degree. relative to top dead center (TDC given by the crank angle of 0.degree.) so that explosive combustion of the fuel gas mixture may occur at a crank angle of about 10.degree. to 60.degree. after passing top dead center (TDC). The knocking due to abnormal combustion will thus occur at the timing falling within the crank angle range of 10.degree. to 60.degree. in succession to top dead center.
Accordingly, upon every occurrence of vibration noise of the cylinders and inter alia knocking, the output signal A of the knocking sensor 1 produced at corresponding periodical intervals assumes a significantly increased amplitude, as can be seen in the waveform shown in FIG. 6 at (a).
In the meanwhile, the engine control unit (ECU) 4 outputs to the gate 22 a masking pulse signal M which is inverted periodically at predetermined intervals in order to ensure that the knocking detection circuit 2 can efficiently receive and process the sensor output signal A. More specifically, the maksing pulse signal M is generated in such a waveform in which the leading edge thereof takes place at a time point corresponding to a crank angle of about 75.degree. advancing relative to the top dead center of the associated cylinder (this advanced angle will hereinafter be represented by affixing "B" to the angle value, e.g. by "B75.degree.") while the trailing edge of the masking pulse M occurs around a time point of B5.degree. (i.e. at a time point corresponding to a crank angle of 5.degree. before TDC), as can be seen in the waveform shown at (b) in FIG. 6. During the period in which the masking pulse assumes the level of "H", the gate 22 is blocked or disabled. Further, as mentioned previously, a reset signal R is outputted to the integrator 25 from the engine control unit 4 periodically at a predetermined timing which coincides with that of the leading edge of the masking pulse signal M.
The filter 21 constituting a part of the knock detection circuit 2 has such a filtering characteristic that the frequency components of the sensor output signal A produced upon occurrence of knocking can pass therethrough, while the gate 22 allows the sensor output signal A to pass therethrough only during a period in which the masking pulse signal M is at the level of "L", as shown at (c) in FIG. 6. The output of the gate 22 is denoted by a reference symbol A'. On the other hand, the background level (BGL) generator 23 generates a background level BGL contained in the gate output signal A' by discriminatively separating the former from the latter, as is illustrated at (d) in FIG. 6, wherein the background level BGL serves as a reference for detection of a knocking event or phenomenon.
When the gate output signal A' exceeds the background level BGL, the comparator 24 determines that knocking has taken place and produces a comparison output of "H" level. The integrator 25 starts to integrate the output signal of the comparator 24 every time it is reset by the reset signal R supplied from the engine control unit 4, as is illustrated at (e) in FIG. 6. The output signal of the integrator 25 is then converted form analog into digital form by the A/D converter 3, the resulting digital integration value V.sub.R being then inputted to the engine control unit (ECU) 4.
In this manner, the engine control unit 4 fetches therein the A/D converted integration value V.sub.R upon every occurrence of ignitions in the cylinder, to thereby generate a retard control angle signal .theta..sub.R for controlling the ignition timing in a sense to suppress knocking. To this end, the retard angle calculator 45 constituting a part of the engine control unit 4 accumulatively adds a current retard angle deviation .DELTA..theta..sub.R to the preceding retard control angle .theta..sub.R * used in the immediately preceding ignition control cycle to thereby generate a current retard control angle signal .theta..sub.R. Accordingly, the current retard control angle .theta..sub.R can be given by the following expression: EQU .theta..sub.R =.theta..sub.R *+.DELTA..theta..sub.R ( 1)
In the above expression (1), the current retard angle deviation (i.e. increment or decrement) .DELTA..theta..sub.R is given by the following equation: EQU .DELTA..theta..sub.R =V.sub.R .times.L
where L represents a weighing coefficient.
In conjunction with the knocking control or suppressing system described above, it is however noted that the knocking sensor 1 is susceptible to failure or fault due to short-circuiting, disconnection (or breakage) or the like. In such cases, there becomes no more available any meaningful sensor output signal A, making it impossible to identify discriminatively an occurrence of knocking, which would ultimately bring about a dangerous state of the internal combustion engine. Accordingly, it is necessary to detect the failure of the knocking detection sensor 1 for thereby effecting a precautionary or safety ignition timing retarding control upon detection of a failure of the knocking detection sensor 1.
With a view to coping with the undersirable situation mentioned above, there is hitherto known a so-called fail-safe ignition timing retarding control scheme in which when the background level BGL detected in the manner described hereinbefore assumes an abnormal value (e.g., of approximately zero level), it is determined that the knocking sensor 1 has failed, whereby a precautionary or safety ignition timing control is put into effect. As other measures for coping with the failure of the knocking sensor, it is also known to connect in advance the signal line for the knocking sensor output signal A to a power supply source via a resister (not shown in FIG. 5) and determine that there takes place a short-circuit failure of the knocking sensor 1 when the potential on the signal line becomes zero while deciding that disconnection or breakage failure takes place when the potential on the signal line assumes that of the power supply source.
As will now be understood from the foregoing, the engine knocking control method and system known heretofore suffers from disadvantages that additional hardware is required for detecting the failure of the knocking sensor 1 and that the ignition timing retarding control might be performed even upon erroneous detection of the knocking sensor failure, because no sensor failure detection means is incorporated in the engine control unit 4.